Summary of work: 1H magnetic resonance spectroscopy (MRS) demonstrated significant increases in concentrations of myo-inositol (50%-60%) and choline-containing compounds (40%-50%) in occipital and parietal brain regions of nondemented adult Down's syndrome (DS) subjects; the neuronal marker N-acetylaspartate was decreased (10%-20%) in occipital cortex. These findings suggest abnormal signal transduction involving phosphatidylinositol and phospholipase C in DS. Global brain concentrations of phosphorus metabolites (ATP, ADP, Pi, mono and di-phosphoesters) were unchanged in Alzheimer's disease (AD) patients using 31P MRS, but the possibility remains for local changes in the brain regions which demonstrate reduced glucose metabolism on positron emission tomography.